Adsorptive bubble separation (which includes froth flotation, bubble fractionation, dissolved air flotation and solvent sublation) is a process in which a molecular, colloidal or particulate material is selectively adsorbed to the surface of gas bubbles that rise through a liquid, and is thereby concentrated or separated. A commonly used type of adsorptive bubble separation process is froth flotation wherein bubble-particle agglomerates accumulate on the liquid surface as a floating froth. The froth with adsorbed (i.e., attached or collected) particles is treated in one of several ways to collapse the froth and isolate the particles. See, for example, Flotation Science and Engineering, K. A. Mattis, Editor, pages 1-44, Marcel Dekker, New York, N.Y., 1995; and Adsorptive Bubble Separation Techniques, Robert Lemlich, Editor, pages 1-5, Academic Press, New York, N.Y., 1972.
This important process is commercially utilized in a wide range of applications including: isolation of minerals and metals from an ore-water slurry, dewatering of microalgae, yeast or bacterial cells, removal of oil from water, removal of ash from coal, removal of particles in waste-water treatment streams, purification of drinking water, and removal of ink and adhesives during paper recycling. In many applications, it is necessary to add reagents, known as “collectors”, which selectively render one or more of the species of particles in the feed hydrophobic, thereby assisting in the process of collection by the gas bubbles. Also, frothing agents may be added to assist in the formation of a stable froth on the surface of the liquid. The process of admitting these various reagents to the system is known as conditioning.
In biochemical process engineering, adsorptive bubble separation finds utility in isolation or concentration of valuable natural products such as are produced by, for example, microalgae. Often in such applications the desired organism or biochemical product is present in very low concentrations. In such cases it is necessary therefore to feed large volumes of a very dilute aqueous dispersion of the desired material through an adsorptive bubble separation process. See, e.g., “Harvesting of Algae by Froth Flotation,” G. V. Levin, et al., Applied and Environmental Microbiology, volume 10, pages 169-175 (1962). U.S. Pat. Nos. 5,776,349 and 5,951,875, the contents of each of which are incorporated herein by this reference, disclose the use of a Jameson cell for dewatering an aqueous dispersion of ruptured microalgae cells.
The particle feed for the adsorptive bubble separation process may be a mixture, dispersion, emulsion, slurry, or suspension of a molecular, colloidal and/or particulate material in a carrier liquid and is referred to hereafter as the liquid-particle dispersion feed. When the liquid is water, as is usually the case, the feed may be referred to as an aqueous-particle dispersion.
One element of the adsorptive bubble processes is the generation of bubbles, typically done by the introduction of a gas into a liquid. The efficiency of an adsorptive bubble process depends on the bubble surface area available for contact with the hydrophobic particles. Small bubbles (of a foam) have a greater surface area available for contact with hydrophobic particles than larger bubbles for a given volume of gas. However, bubbles that are too small will not rise within the carrier liquid until they coalesce, which takes time. Thus, a balance exists between the size of the bubbles and the size of the separation zone required to give the time needed for the bubbles to rise within the carrier liquid. Bubbles from 0.5 to 2 millimeters have been shown to work well within the art, although the characteristics of the carrier liquid and the hydrophobic particles can influence the design. Consistently generating bubbles of the optimum size and in sufficient quantity has been a challenging area of continued study, and many methods have been proposed and tested.
Having generated the bubbles, the hydrophobic particles are brought into contact with them with sufficient probability and energy that the hydrophobic particles penetrate the liquid boundary layer surrounding the bubbles and attach to the bubble surface. A dense bubble population results in thin films of the carrier liquid between the bubbles and greater opportunity for interaction and collection. The population of bubbles within a liquid is referred to as the “void fraction”, and high void fractions favor the collection of hydrophobic particles, as there are many bubbles, and the layers surrounding them become thinner and attachment easier.
Because of the importance of adsorptive bubble separation processes, there have been many attempts to improve the efficiency and selectivity of particle capture from an aqueous-particle dispersion in order to increase product yield and purity. In all of these applications, a need exists to efficiently contact particles or droplets from a liquid dispersion with a gas and then attach the hydrophobic particles to the bubbles.
Presently columnar adsorptive bubble separation process systems fall within two broad categories—(I) those where the gas and feed flow counter-currently in a columnar system, and (II) those where the gas and feed flow co-currently, as in a compact, Jameson-type cell.
In columnar systems, the collection and separation zones are combined into a single large, tall cylindrical tank or column. The liquid-particle dispersion feed is introduced near the top of the column, and the hydrophobic particles tend to settle downward. Pressurized air or aerated liquid is introduced near the bottom forming a rising bubble flow. The rising bubbles and the settling particles must collide with enough energy for the particles to penetrate the boundary layer surrounding the bubbles and attach. As this relies on the probability of collision and energy being sufficient, the columns must be quite tall to increase the time of exposure and thus the recovery. In these counter-current flow contacting devices, a long residence time is needed to facilitate sufficient bubble-particle collision probability, making the technology relatively expensive to practice.
Examples of the counter-current columnar system include the MICROCEL™ flotation column, described in U.S. Pat. Nos. 5,167,798 and 5,078,921 as well as the Chiang Column, U.S. Pat. No. 5,897,772, the contents of each of which are incorporated herein by this reference. Similar systems include mechanical flotation cells, such as those described in U.S. Pat. No. 5,205,926, the contents of which are incorporated herein by this reference, wherein a mechanical stirring mechanism within a tank agitates the carrier liquid, the hydrophobic particles, and the gas to form the bubble generation zone. The hydrophobic particles and the bubbles are mixed in the region surrounding the impeller; where the hydrophobic particles and the bubbles collide to form hydrophobic particle-bubble agglomerates. Like the standard columnar systems, these mechanical flotation cells combine the bubble generation, contacting, collection, separation, and froth zones within the same vessel, thereby compromising the efficiency of the flotation cell, requiring a number of flotation cells operated in series in order to achieve the desired efficiency. Further, columnar systems are typically quite large and therefore impractical to move and store.
Co-current systems, which are typically based upon a Jameson cell design, are compact systems wherein feed is used to generate the foam and bubbles by means of a downward plunging jet in a downcomer, with all of the feed being fed through the jet to generate the foam/bubbles. Examples of these systems include those described in U.S. Pat. Nos. 5,332,100, 4,938,865, 4,668,382, 5,776,349, 6,092,667, WO 2006/056018 A1, and WO 2006/056018 A1 (the contents of the entirety of each of which are incorporated herein by this reference). The foam/bubbles are released into the flotation chamber through a duct or similar means, where the bubbles coalesce and disengage from the liquids, and rise to the top of the chamber to form a froth. Because adsorptive bubble processes are commonly used to separate low concentrations of hydrophobic particles from a carrier fluid, large volumes of fluid must be injected through the jet in these compact systems. Therefore, a substantial amount of power is required to charge the liquid-particle dispersion feed and the gas to the adsorptive bubble process. This power input can be a dominant cost when the adsorptive bubble process is used for the recovery of dilute material with a relatively low value, such as when algae are being dewatered for the production of biofuels.